DLFs for the tDCS group returned to baseline levels between sessions while remaining at trained levels for the sham group, suggesting that stimulation degraded the consolidation of learning. This is unlike the effect of motor skill learning where anodal tDCS increases between-day consolidation (Reis et al., 2009). There is evidence from letter enumeration
tasks, where subjects determine if the number of letters presented is odd or even, showing that learning is only retained if asymptotic performance is reached within each session (Hauptmann & Karni, 2002; Hauptmann et al., 2005). In the current study, the sham group had stable performance between Blocks 2 and 3, whereas DLFs for the tDCS group decreased in this period, suggesting asymptotic thresholds had not been reached Panobinostat chemical structure in the session. The lack of effect of tDCS on frequency selectivity around 1000 Hz, and the decreased sensitivity to TFS during tDCS, indicate that the degradation of frequency discrimination around 1000 Hz by anodal tDCS was probably due to interference with temporal coding. Imposing a transcortical DC current has been shown to immediately alter the spontaneous firing rate of cortical neurons in the rat (Bindman et al., 1964) and it is possible that tDCS interferes directly with temporal
coding by disrupting the precision of the phase-locked firing selleck compound pattern of active auditory neurons. Most evidence from both animals and humans points to the importance of temporal coding for frequency perception below 4000–5000 Hz (Rose Wilson disease protein et al., 1967; Johnson, 1980; Moore, 2012). Auditory neurons show millisecond-precise phase-locking firing rates to both complex and pure tone frequencies below 5000 Hz (Zatorre, 1988; Averbeck et al., 2006). Even small perturbations of temporal coding, in the scale of milliseconds, result in information loss for cortical neurons (Kayser et al., 2010). Changing the excitability of auditory cortex using tDCS could therefore sufficiently disrupt the fine structure information needed for precise temporal
coding. There do not appear to be any studies in either animals or humans showing a dissociation of place and temporal coding processes following lesions to auditory cortex, although bilateral lesions impair perceptual discriminations relying on both temporal (Bowen et al., 2003) and place (Cooke et al., 2007) coding. These processes do appear to be at least partially lateralized, with the left hemisphere showing a preference for temporal information and the right showing a preference for place information (Zatorre & Belin, 2001; Schönwiesner et al., 2005). Our findings that anodal tDCS over auditory cortex decreased frequency selectivity at 2000 Hz but not at 1000 Hz, and decreased sensitivity to temporal fine structure, show that altering auditory cortical excitability in this way has complex effects on auditory function.